Unanticipated Stickiness of α‑Pinene

The
adsorption of α-pinene to solid surfaces is an important
primary step during the chemical conversion of this common terpene
over mesoporous materials, as well as during the formation of atmospheric
aerosols. We provide evidence of tight and loose physisorbed states
of α-pinene bound on amorphous SiO<sub>2</sub> as determined
by their adsorption entropy, enthalpy, and binding free energies characterized
by computational modeling and vibrational sum frequency generation
(SFG) spectroscopy. We find that adsorption is partially (40–60%)
irreversible over days at 294–342 K and 1 ATM total pressure
of helium, which is supported by molecular dynamics (MD) simulations.
The distribution of α-pinene orientation remains invariant with
temperature and partial pressure of α-pinene. Using the Redlich–Peterson
adsorption model in conjunction with a van’t Hoff analysis
of adsorption isotherms recorded for up to 2.6 Torr α-pinene
in 1 ATM total pressure of helium, we obtain Δ<i>S</i>°<sub>ads</sub>, Δ<i>H</i>°<sub>ads</sub>, and Δ<i>G</i>°<sub>ads</sub> values of −57
(±7) J mol<sup>–1</sup> K<sup>–1</sup>, −39
(±2) kJ mol<sup>–1</sup>, and −22 (±5) kJ
mol<sup>–1</sup>, respectively, associated with the reversibly
bound population of α-pinene. These values are in good agreement
with density functional theory (DFT)-corrected force field calculations
based on configurational sampling from MD simulations. Our findings
are expected to have direct implications on the conversion of terpenes
by silica-based catalysts and for the synthesis of secondary organic
aerosol (SOA) in atmospheric chambers and flow tubes.